Answer:
The time after which only 40 mg of medicine left inside body is 9.8 hours
Explanation:
Given as :
The initial quantity of medicine ingest in body = i=200 mg
The final quantity of medicine in body = f= 40 mg
The rate at which body remove medicine = r = 15%
Let The time taken to remove = t hours
According to question
The final quantity of medicine in body after t hours = The initial quantity of medicine ingest in body ×

I.e f = i ×

Or, 40 mg = 200 mg ×

Or,
=

Or , 0.2 =

Or,
= 0.2
Taking Log both side
So,
=
0.2
Or, t ×
0.85 =
0.2
Or, t (-0.07) = - 0.69
∴ t =

I.e t = 9.8 hours
So, The time after which only 40 mg left inside body = t = 9.8 hours
Hence,The time after which only 40 mg of medicine left inside body is 9.8 hours .Answer